The disclosed seal is used in a high performance gas turbine engine as a shaft seal for a shaft rotating at 465 feet per second for the purpose of preventing gases at high temperature (1200.degree. F.) and pressure (45 psia) from entering the engine oil system. Under these conditions prior art seals, capable of preventing oil of air leakage, had very short lives.
My prior co-invention, U.S. Pat. No. 3,964,753, issued June 22, 1976, was an improvement over the prior art in that it utilized a carbon ring which under normal operating conditions was subject to very little wear. The carbon seal ring was fabricated with a single crack and so that its inner diameter is initially larger than the cool seal runner size, but slightly smaller than the hot runner size. Therefore, the carbon seal ring initially presented a gap through which controlled air leakage occurred. However, when the shaft diameter grew due to temperature and centrifugal forces during initial operation, the carbon seal ring was worn by runner friction until its final size was equal to the hot seal runner diameter. Thus, after initial operation the carbon seal ring was perfectly conformed to the seal runner and has essentially no gap, only an oil bearing film was provided between the carbon seal ring and the shaft. The carbon ring was fractured at one location to permit ring expansion sufficient to accommodate initial thermal growth of the shaft. The carbon ring was pinned to the housing through a single radial slot for anti-rotation of the carbon ring and to accommodate shaft eccentricity or run out.
The present invention is an improvement over U.S. Pat. No. 3,964,753 in that I eliminate the anti-rotation pin, and instead I permit a limited rotation, the amount of which is controlled by means of drag pad integral with the ring. The drag pad consits of a radial enlargement of the seal face along the periphery of a small arc of the ring. The drag pad provides increased friction especially at one end of the ring and hence limits the speed of rotation of the ring, and provides an unwinding action to the carbon ring.